A paint composition is often applied onto the surface portion of a golf ball in order to protect the ball surface or to maintain an attractive appearance. Two-part curing polyurethane paints which are used by mixing together a polyol and a polyisocyanate just prior to painting are suitably employed as such golf ball paint compositions for a number of reasons, including their ability to withstand large deformation, impact and abrasion (see, for example, JP-A 2003-253201).
A frequent goal of recent golf ball development has been to further lower the spin rate on full shots taken with a driver. This move toward lower spin rates has engendered a trend toward greater hardness in the cover serving as the outermost layer of the ball.
Most golf balls have a core, a cover positioned outside of the core, and a paint film layer positioned outside of the cover. Making this paint film layer soft also often provides certain advantages, such as contributing to stability in the spin rate of the golf ball and imparting outstanding durability (see, for example, JP-A 2011-67595).
However, JP-A 2011-67595 does not take into account the scratch performance of the paint film layer. When a scratch forms in the paint film layer, the paint film has a tendency to peel from the affected area and the durability of the paint film may worsen. Also, the scratchability of the paint film on a golf ball is known to be affected by the underlying cover material; the higher the cover hardness, the greater the tendency for the scratch resistance to decrease and the paint film durability to worsen. What the golfer desires most in a golf ball is distance, and so, in many golf balls, properties such as the hardnesses and thicknesses of the core and cover are adjusted with increased distance as the primary aim. Golfers attach particular importance to the carry of the ball on shots taken with a driver. The carry of the ball on a driver shot correlates to the rebound performance of the golf ball; many golf balls in which the rebound performance has been increased by making use of a relatively hard cover are sold on the market. Therefore, in golf balls having a paint film layer, the relationship between the paint film layer and a hard cover layer is important. A major challenge has been how to not only improve the scratch performance and durability of the ball surface, but also deliver an excellent distance on shots with a driver and the like.